Typescript gen : invalid SQL Function types

# Bug report



- [x] I confirm this is a bug with Supabase, not with my own application. 
- [x] I confirm I have searched the [Docs](https://docs.supabase.com), GitHub [Discussions](https://github.com/supabase/supabase/discussions), and [Discord](https://discord.supabase.com). 

## Describe the bug

For both input and input types, typescript generation of SQL functions is invalid in some cases :
- Primitive types are generated as non-nullable (`type` instead of `type | null`) even though the function may accept NULL as input or output
- Domain types are generated as `unknown`

I think that this bug is not caused by a PostgreSQL limitation as views produce the correct typing : primitives are nullable, domains generate as their underlying type and when domains are marked `NOT NULL` they are even non-nullable in typescript.

## To Reproduce

```sql
CREATE DOMAIN strict_text AS text NOT NULL;

CREATE FUNCTION some_function(arg strict_text)
RETURNS table (nulltext text, stricttext strict_text)
LANGUAGE SQL AS $$
   SELECT NULL::uuid, arg
$$;
```

Generated type with `supabase gen types typescript --local`
```typescript
export interface Database {
  public: {
    Functions: {
      some_function: {
        Args: {
          arg: unknown
        }
        Returns: {
          nulltext: string
          stricttext: unknown
        }[]
      }
    }
  }
}
```

## Expected behavior

Generated types should be
```typescript
export interface Database {
  public: {
    Functions: {
      some_function: {
        Args: {
          arg: string
        }
        Returns: {
          nulltext: string | null
          stricttext: string
        }[]
      }
    }
  }
}
```

## System information

- OS: [e.g. Linux NixOS]
- Version of supabase CLI: 1.50.8
- Version of Node.js: 18.15.0

## Additional context

I :heart: the work being done and the mindset at Supabase. I believe strict typing is crucial to the success of a "backend in the DB" and I could help with a PR if I'm being given some pointers to the code responsible of the TS generation :wave:

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